High-Temperature Oxidation of Alpha Nickel-Silicon Alloys. Microstructures and Mechanisms,
Abstract
The subject of this report is the metallography of a series of nickel and nickel-silicon alloys which have been isothermally oxidized at 1373k in static air for up to 164h. The aim of the study was to determine the mechanisms of oxidation that produced the morphology and microstructures of these oxidized alloys. Pure nickel, after surface preparation by tumbling, oxidizes by outward lattice diffusion of nickel cations and inward gain boundary and other short circuit path diffusion of oxygen. Gain boundary oxides accounted for about 15% of the total weight gained by nickel during oxidation. Additions of 1% and 2% Si to nickel decreased the oxidation rate compared with pure nickel by reacting with the inwardly diffusing oxygen to prevent it diffusing deeply down the grain boundaries of the alloys (the major effect) and by retarding the outward flow of nickel cations (the minor effect). The 3%, 4% and 5% Si alloys form external oxides and oxidize at markedly slower rates than the alloys with lower concentrations of silicon. While the formation of Silicon dioxide significantly decreases the isothermal oxidation rate it also causes the oxide scale to spall upon cooling because of the difference in thermal expansion coefficients between silicon dioxide and the alloys. Under cyclic oxidation conditions, externally oxidizing NiSi alloys will not be protective unless alloy additions are made to improve the adherence of the oxide scale. Keywords: Heat resistant alloys.
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 01, 1986
- Accession Number
- ADA176211
Entities
People
- J. L. Cocking
Organizations
- Defence Science and Technology Group